1. Technical Field
The invention relates to network technology applied to form a ring network. The network technology includes a network device, a network system including the network device and a computer program product configured to control the network device. The network device represents a hardware component to form a network interconnecting computers and may be, for example, router, switch, hub, or network interface card.
2. Description of the Related Art
In a layer 2 network, when a failure is detected in some line of the network or in a network device, the general control procedure changes the communication route to an alternative route provided in advance, thus ensuring the redundancy of the network. The layer 2 network with the redundancy has the loop structure. In order to avoid the routing loop, part of the network device included in the layer 2 network is set in the state where forwarding of user traffic is not allowed.
One proposed protocol to control the prohibition or the permission for forwarding the user traffic is specifically applied to a ring network, in which a plurality of network devices are arranged in a ring shape. One of the network devices forming the ring network is specified as a master node, while the other network devices are specified as transit nodes. The master node monitor the status of the ring network by periodically sending a control frame from one of ring ports and receiving the control frame at the other of the ring ports. In the state where the control frame is received at the other ring port, the master node determines that the ring network has the loop structure, and prohibits forwarding the user traffic. In the state where the control frame is not received at the other ring port, on the other hand, the master node determines that a failure occurs in the ring network, cancels prohibition of forwarding the user traffic and changes the forwarding route of the user traffic to an alternative route going through the master node. This ensures the redundancy of the network. The protocol to control the ring network by this method is called “ring protocol”.
In the ring network, in order to meet the future need for bandwidth expansion, there may be a requirement to replace an existing network device with a new network device that supports the line for ultra-high transmission rate. The following problem may arise during replacement of the existing network device serving as the master node with a new network device.
The general replacement operation of the master node detaches the old master node as the object to be replaced from the ring network and subsequently attaches a new master node to the ring network. This operation does not allow the status of the ring network to be monitored with control frames and thereby prevents detection of the occurrence of a failure in the ring network for a time period between the detachment of the old master node and the attachment of the new master node.